Hardness is a fundamental material parameter that measures the resistance of a material to an applied compressive load. It is a function of both the strength of the interatomic bonding and of the rigidity (or compliance) of the lattice framework. Diamond is the hardest known bulk material (approximately 70 GPa "giga pascals"), due to strong covalent sp.sup.3 bonding in a tetrahedral lattice configuration, and its hardness would make it an excellent material for grinding ferrous-based materials such as engine blocks and other automotive components. However, it is expensive and reacts with iron (graphitization) leading to rapid wear when applied against ferrous and high nickel-content alloys. Cubic boron nitride, .beta.-BN, is a material with the diamond crystal structure in which the carbon atoms of diamond are replaced by boron and nitrogen. The resulting material exhibits much lower chemical reactivity with iron, but also possesses a significantly lower hardness than diamond, approximately 45 GPa. The cubic polymorph of BN can be prepared only by a combination of high temperatures (1800 to 2000.degree. C.) and extremely high pressures (65 kbar=50,000 atmospheres). As a result, the cost of .beta.-BN can exceed $7,000 per pound ($15,000 per kg). Consequently, .beta.-BN is prohibitively expensive for all but the most high value, specialized applications. For example, grinding and finishing operations for automotive hardened steel stamping dies are constrained to the use of .beta.-BN for milling applications due to lack of a suitable cost-effective alternative. The next hardest abrasive bulk materials are silicon carbide and vanadium carbide, each with a hardness within the 24 to 28 GPa range. The lower hardness of these materials renders them inappropriate for the demands of high volume, industrial grinding and finishing operations. Therefore, development of a cost-effective abrasive material with a hardness comparable to that of .beta.-BN would be a significant benefit to industry for which there is a real need.
A recent study of complex ternary borides revealed a new class of lightweight, ultra-hard ceramics. These alloys, aluminum magnesium boride alloyed with a few atomic percent group IV or group V element (AlMgB.sub.14 :X where X=Si, P, C), were prepared by mechanical alloying, a high energy solid state technique, and consolidated by vacuum hot pressing. The AlMgB.sub.14 intermetallic compound is based on four B.sub.12 icosahedral units positioned within an orthorhombic unit cell containing 64 atoms. The icosahedra are positioned at (0,0,0), (0,1/2,1/2), (1/2,0,0), and (1/2,1/2,1/2) while the Al atoms occupy a four-fold position at (1/4,3/4,1/4) and the Mg atoms occupy a four-fold position at (0.25,0.359,0). The unique electronic, optical, and mechanical properties of this material are due to a complex interaction within each icosahedron (intrahedral bonding) combined with interaction between the icosahedra (intericosahedral bonding). The hexagonal icosahedra are arranged in distorted, close-packed layers. Table I provides a comparison with several hard materials along with their corresponding density, bulk, and shear moduli.
TABLE I ______________________________________ Density, Hardness, Bulk and Shear Moduli of Selected Hard Materials Bulk Shear Density Hardness Modulus Modulus (g/cm.sup.3) (GPa) (GPa) (GPa) ______________________________________ C (diamond) 3.52 70-90 443 535 BN (cubic) 3.48 50-60 400 409 C.sub.3 N.sub.4 (cubic) .dagger. .dagger. 496 332 SiC 3.22 24-28 226 196 Al.sub.2 O.sub.3 3.98 21-22 246 162 TiB.sub.2 4.50 30-33 244 263 WC 15.72 23-30 421 -- AlN 3.26 12 203 128 TiC 4.93 28-29 241 188 AlB.sub.12 2.58 26 Si.sub.3 N.sub.4 3.19 17-21 249 123 AlMgB.sub.14 2.66 35-40 * * ______________________________________ *unknown, or not well characterized .backslash. presently available in quantities too small to permit measurement of density and hardness
Prior work on these complex orthorhombic borides has mainly involved determination of crystal structure. I. A. Bairamashvili, L. I. Kekelidze, O. A. Golikova, and V. M. Orlov J. Less Comm. Met. 67 (1979) 461 initially examined the thermoelectric properties of this and related borides prepared by hot pressing powders produced from crystallization of aluminum melt solutions. They observed that these compounds exhibited high melting points and were relatively brittle. W. Higashi and T. Ito J. Less Comm. Met. 92(1983)239 conducted an extensive crystallographic study on the 1:1:14 compound and established the lattice parameters and atom positions but performed no property measurements. More recently, H. Werheit, U. Kuhlmann, G. Krach, I. Higashi, T. Lundstrom, and Y. Yu J. Alloys and Compounds 202(1993)269 examined the optical and electronic properties of the orthorhombic AlMgB.sub.14 prepared by growing single crystals in alumina crucibles from Al-B solutions containing Li or Mg. In addition to their unique mechanical properties, evidence suggests that these systems exhibit novel electronic properties such as hopping conduction. Crystallographic studies indicate that the metal sites are not fully occupied in the lattice so that the true chemical formula may be closer to Al.sub.0.75 Mg.sub.0.78 B.sub.14, which is contemplated by the formula here used as AlMgB.sub.14.
The primary objective of this invention is to provide a new, lightweight, extremely hard ceramic by intentionally modifying the composition of the baseline alloy.